Hi there:
You have some great planning tools available on Walt Disney World's website.
I would start by visiting Disney's
in-depth planning page for planning your first trip. Order
Disney's Vacation Planing DVD (look for a quick cameo from the Disney Moms), browse
Magic Your Way vacation packages and take advantage of Disney's "
My Vacation" planning tool.
Another popular site, allears.net, which is not an official Disney website, also has some great planning tools. Just use the search function on the website to search for "planning."
Don't worry about doing everything. Disney World is so large and there are so many activities that it would take many weeks to come close to doing everything. The best advice is to plan, but make sure to also relax, follow your twins' pace, and enjoy. When you get to the Magic Kingdom, check in at City Hall, or at guest relations at other parks, and pick up "first-visit" buttons. Many Cast Members will go out of their way to greet your family with a kind gesture.
Enjoy your vacation.